Two conductive lines are coupled when they are spaced apart, but spaced closely enough together for energy flowing in one to be induced in the other. The amount of energy flowing between the lines is related to the dielectric medium the conductors are in and the spacing between the lines. Even though electromagnetic fields surrounding the lines are theoretically infinite, lines are often referred to as being closely or tightly coupled, loosely coupled, or uncoupled, based on the relative amount of coupling.
Parallel transmission lines couple both electrically and magnetically. The coupling is inherently proportional to frequency, and the directivity can be high if the magnetic and electric couplings are equal.
For edge coupling between two planar conductors, it may be sufficient that the conductors have facing edges, and for broadside coupling, it may be sufficient that the conductors have facing broad surfaces. Two faces may be considered facing, for instance, if a line can be drawn directly between them. Correspondingly, two faces may be considered overlapping if a line normal to the face of one conductor intersects a face of another. Surfaces may thus be facing each other without being overlapping or directly opposite each other.